JP 2005-070809 A discloses a stereomicroscope having a main observer beam path and a plurality of co-observer beam paths. In the stereomicroscope there exist a co-observer beam path which is coupled out from the main observer beam path in front of the main objective, that is, between main objective and observation object, and co-observer beam paths which are coupled out behind the main objective. The latter share with the main observer beam path not only the main objective but also a zoom system including dedicated lenses for each stereoscopic partial beam path. Binocular tubes for the co-observer beam paths can be connected to interfaces of the microscope housing that are provided for this purpose. In this case, there are interfaces for face-to-face positioning of main observer and co-observer, which involves main observer and co-observer being situated opposite one another, and interfaces for lateral positioning of the co-observer, which involves main observer and co-observer being situated at an angle of 90 degrees with respect to one another. Other co-observer positions are not possible in the case of the co-observer beam paths.
The co-observer beam path which is coupled out between main objective and observation object in the stereomicroscope from JP 2005-070809 A can be rotated about the optical axis of the main objective of the main observer beam path, such that arbitrary positioning of the co-observer relative to the main observer is possible, in principle. However, since this co-observer beam path is already coupled out in front of the main objective of the main observer beam path, it necessitates a dedicated main objective, which increases the complexity and, in association therewith, the production costs for the co-observer beam path. In addition, the beam splitter arranged between the observation object and the main objective results in loss of light in the main observer beam path and thus in a reduction of the image quality for the main observer. Finally, the stereomicroscope described in JP 2005-070809 A takes up a relatively large amount of space if the co-observer beam path which is coupled out between the main objective of the main observer beam path and the observation object is used.
U.S. Pat. No. 6,304,374 B1 describes a stereomicroscope including a main objective, a zoom system, a main observer beam path and a co-observer beam path. The co-observer beam path therein is coupled out from the main observer beam path behind the zoom system, such that the main observer beam path and the co-observer beam path share not only the main objective but also the zoom system. The zoom system in U.S. Pat. No. 6,304,374 B1 is embodied as a large zoom system, that is, the lens diameters of the zoom system are of a magnitude such that both stereoscopic partial beam paths of the main observer beam path pass through each of the lenses. In comparison with a zoom system in which a dedicated partial zoom system is present for each partial beam path, a zoom system through which both stereoscopic partial beam paths pass must have a larger dimensioning in the direction of the optical axis, referred to hereinafter as longitudinal dimensioning, since otherwise the requirements made of the precision of the lenses used and the precision of the alignment increase to such a great extent that the production costs are significantly higher than with the use of separate zoom systems for the stereoscopic partial beam paths. However, the large zoom system affords the possibility of being able to freely choose the orientation of the stereo base, that is, the orientation of the connecting line between the midpoints of the stereoscopic partial beams within a plane perpendicular to the optical axis, even after passage through the zoom system. If this is intended to be possible for a co-observer as well, the co-observer beam path behind the zoom system must be coupled out from the main observer beam path via a beam splitter that is large enough that the stereoscopic partial beam paths can be coupled out from the main observer beam path in any orientation of the stereo base. This results in loss of light in the main observer beam path and thus in a reduction of the image quality for the main observer.
The possibility of freely orienting the co-observer beam path in a large angular range is thus purchased at the expense of a reduction in the image quality for the main observer in JP 2005-070809 A and in U.S. Pat. No. 6,304,374 B1.